Movements of the Moon - Part I

Even though I'm a geologist by training, I do teach courses on Observational Astronomy and Ancient Astronomy (both offered this fall semester, the Observational Astronomy on Thursday nights and the Ancient Astronomy online - contact me if you're interested!).

Teaching these courses has made me much more in tune with the sky and one of the things I always find myself aware of is the phase of the Moon.  As I write this here in Ulster County, for example, it's after midnight and the nearly full Moon is lighting up the night outside.

Over the next few days, I'll be posting some information about the movements of the Moon and I'll start with the most obvious things and then move on to the more subtle movements.  Let's start with the phases of the Moon.

Moon "phases" refer to the percentage and portion of the Moon illuminated by the Sun as seen from the Earth's surface.  A New Moon is not illuminated and invisible.  A Full Moon is 100% illuminated.  The time it takes to go from the New Moon to the Full Moon and back to the New Moon is one cycle of phases and takes about a month (the words "moon" and "month" are obviously cognates).

As the Moon becomes more illuminated, we say it's waxing and as it become less illuminated it's waning.  As the moon waxes, the illumination starts on the right-hand side and progresses from day-to-day until the Moon is full (at least in the Northern Hemisphere).  When the Moon wanes, the the illuminated portion becomes smaller and smaller on the left side of the Moon.

A thin sliver of the Moon is called a crescent, when half the Moon is illuminated we call it a Quarter Moon (1st quarter for waxing and 3rd or last quarter for waning), and when the Moon is mostly illuminated it's gibbous (from the Latin gibbus meaning "hump").  Why quarter Moon when it's half lit?  It's from the fact that they occur at the 1/4 and 3/4 points of the Moon's cycle of phases.

Picture the image below to remember the sequence (read the story behind this).

As I've said, this cycle of phases (called a lunation) takes about a month.  More precisely, it takes 29.53 days (29 days, 12 hours, 44 minutes, and 3 seconds) on average.  Why "on average"?  Because it can vary from lunation to lunation by several hours (approximately 29.18 to 29.93 days) since the Moon's orbit around the Earth is an ellipse and the Earth's orbit around the Sun is also an ellipse and not perfect circles (we'll come back to this later).

If you want to get even more precise, an average lunation takes 29.530588853 + 0.000000002162 * Y days where Y is the number of years since January 1, 2000.  You can see from this formula that the average length of each lunation increases by 2 seconds every 10,000 years (0.000000002162 day/yr * 86,400 sec/day = 0.0001867968 sec/yr).

The time it takes for one lunation is called the synodic month (we'll talk about other "kinds" of months in subsequent posts).  The word synodic, by the way, is derived from the Greek σὺν ὁδῴ (sun hodō) meaning "together with [the Sun]."  This is because Moon phases have to do with the Moon lining up with the Earth and the Sun as we'll see tomorrow.

Lunations are sometimes numbered by a system developed by British mathematician and astronomer Ernest William Brown (1866-1938).  Lunation 1 was the first new Moon in 1923 (0241 UTC, January 17, 1923), the year his lunar tables were incorporated into astronomical almanacs - we're currently in lunation number 1095.

OK, so we know the Moon goes through a cycle of phases - New, Waxing Crescent, 1st Quarter, Waxing Gibbous, Full, Waning Gibbous, 3rd Quarter, Waning Crescent, and New again - in 29.53 days.  Why does it do this?  The full explanation will have to wait until tomorrow.